Plant-geography of the Pacific. As based on a census of Phanerogam Genera

In 1960 I made a preliminary analysis of the floristic distribution of the native Phanerogam genera of the Pacific islands, which amounted to 1511 genera in all. The aims of the present work have been to record these more accurately and more critically in detail, especially with regard to native versus introduced, to complete the survey with new records from new explorations made during the interval, and to evaluate new taxonomic literature on Pacific genera. The present list amounts to a total of 1666 genera, as far as known in July 1969, listed in an Appendix. The floristic relationships of the Pacific islands and the surrounding continental areas are established and a hierarchical subdivision of the flora of the Pacific islands based on demarcations in it is made. Furthermore a nomenclatural stabilization of the names and ranks of the subdivisions is attempted. Chapter IV, 3. An attempt was also made to find factual data on the correlation between distribution and means of dispersal. Chapter IV, 4. Secondary aims were to review earlier attemps towards a subdivision of the Pacific flora (Chapter II), two other secondary purposes to see whether traces of the historic plant-geography of the Pacific flora are still reflected in the present flora (Chapter V), and finally to compare geographic subdivisions and other data from non-Phanerogam taxa, mostly animals, with floristics. Chapter VI. Chapter III is devoted to an explanation and a discussion of the methods employed. Arguments are given why only Phanerogams have been considered and why only native genera have been used for computing results. Chapter III, 1—2. Arguments are given for employing the genus as a working unit. It is shown that the genus is much less susceptible to variability in taxonomic concepts than either the species or the family. Besides it is comparatively easy to establish the distribution of a genus fairly reasonably from literature. Chapter III, 3. Chapter III, 4 is devoted to a discussion on the sources of information on which this work is based, comprising i.a. literature, herbarium collections and personal information. Many errors are contained in the first two of these and it cannot be avoided that some mistakes have not been detected. Also, the island groups have been investigated with a varying degree of intensity. The island groups in the Pacific are taken as geographic units of which there are 36. The surrounding land masses are divided into 12 main areas. Chapter III, 5. Of each genus occurring in any of the 36 Pacific unit areas the full distribution is traced. See Appendix. From a comparative study of generic ranges, it has appeared that they exhibit a restricted number of recognizable patterns, 17 of which have been distinguished. These I have called distribution types in this work. Chapter III, 6. The choice of geographic unit areas introduces a certain element of arbitrariness. Each island group can then be characterized by its set of distribution types: the distribution types spectra. It is also possible to calculate floristic relationships or resemblance between the island groups, for which a number of methods are discussed and evaluated. It appears that basically all methods lead to more or less similar conclusions. Chapter III, 9. As a test for the validity of the conclusions based on the distribution of all genera, similar calculations were performed on 345 revised or otherwise well-known taxa. Although the percentages of the distribution types are slightly different the general conclusions are corroborated. Chapter III, 7. In addition, an attempt has been made to find whether there is a correlation between the distribution and the means of dispersal of these revised or otherwise well-known taxa. Chapter III, 8. One of the most important results of this work is the census of Pacific genera. See Appendix. By using the method of distribution types spectra, demarcation knots and other methods it has been possible to find demarcations and to define phytochores. The main demarcation is that between the New and Old World floras. A hierarchy is set up of subdivisions which is illustrated in fig. 35 and tabulated in table 6. It appears that a strong demarcation exists between the islands on the American side of the Pacific (Galapagos, Juan Fernandez, etc.) and the western islands. Hawaii and SE. Polynesia form the easternmost frontier of the OldWorld flora. This conclusion was reached almost unanimously by all phytogeographers, one of the earliest being Engler after whom I have proposed to name this demarcation: Engler’s line. In the W. Pacific Bonin in the north and New Zealand and adjacent islands in the south show a sharp demarcation from the rest, Bonin forming part of the E. Asiatic region, and New Zealand forming a distinct subregion of the Australian. New Caledonia cannot be satisfactorily placed. It shows relations with New Guinea, Queensland and the Pacific in about equal measure. Besides it abounds in endemics, some of which are highly peculiar in various aspects. The remaining part of the Pacific shows an essentially Malesian character, decreasing in strength from west to east. The New Hebrides with Fiji, Samoa and Tonga form a subprovince as does SE. Polynesia, Hawaii is considered a separate province of the Malesian subregion. Unlike the islands west of Engler’s line the American Pacific islands show very little mutual floristic alliance, but they all have a characteristic American flora. Comparisons with subdivisions and demarcations of other groups of organisms show that often, but not always, the same barriers are respected by unrelated groups. My data give certain indications about the past but no attempt has been made to correlate the conclusions with contemporary geological theories. The regularity of distribution patterns, the close floristic alliance among the islands west of Engler’s line independent of their distance from each other, combined with the fact that dispersal spectra show no clear correlation between distribution and ‘dispersibility’, suggests an old relictual character of the flora rather than a young one built up by random long-distance dispersal. This applies especially to the W. Carolines, the Melanesian islands, Lord Howe I. and New Zealand, i.e. islands more or less within the Andesite line, which are much richer and contain many poor dispersers. For Hawaii also a better accessibility in the past seems indicated. The regular decrease in the number of taxa in proportion to their distance from source areas is discussed. An attempt is made to explain the phenomenon. A tentative conclusion is reached that impoverishment and other phenomena attributed to oceanic islands are not restricted to these. A large scale comparative study of continental and island floras is needed.